1. Field of the Invention
The present invention relates to a rear focusing zoom lens, and, more particularly, to a rear focusing zoom lens for use in a photographing camera, a video camera, a broadcasting camera or the like and exhibiting a high power-variable ratio and aperture ratio, relatively wide angle of view and short overall length such that the variable power ratio is 6 to 20 and the F-number is about 1.6 to 1.8 at a wide angle end thereof.
2. Related Background Art
Recently, the trend of reducing the size and weight of a personal video camera or the like has caused the sizes of photographing lenses to be reduced significantly. In particular, the overall length of the lens and the diameter of the front lens have been intended to be decreased, and the structure has been desired to be simplified.
As a means for achieving the foregoing objects, a so-called rear focusing zoom lens has been known, in which focusing is performed by moving lens units other than the first lens unit near an object.
Generally, in the rear focusing zoom lens, the first group has a small effective aperture as compared with that of a zoom lens of a type in which focusing is performed by moving the first lens unit. Therefore, the size of the overall lens system can easily be reduced, and close-up photographing, in particular, very close-up photographing can easily be performed. Furthermore, since relatively small and light lens units are moved to perform the close-up photographing operation or the very close-up photographing operation, the power required to move the lens units can be reduced, and, therefore, focusing of an image can quickly be performed.
A rear focusing zoom lens of the foregoing type has been disclosed in Japanese Patent Laid-Open No. 5-215967, which comprises five lens units consisting of a first lens unit having positive refracting power that is fixed during the power variation, a second lens unit having negative refracting power for varying the power, a third lens unit having positive refracting power, a fourth lens unit having negative refracting power, and a fifth lens unit having positive refracting power for correcting change in the image surface occurring when the power is varied, the lens units being disposed in the foregoing sequential order when viewed from the object. When the power is varied, the third and/or the fourth lens unit is moved. When focusing is performed, at least any of the third, fourth and the fifth lens unit is moved.
The applicant of the present invention has disclosed a rear focusing zoom lens in Japanese Patent Laid-Open No. 6-148523 which comprises five lens units consisting of a first lens unit having positive refracting power, a second lens unit having negative refracting power, an aperture stop, a third lens unit having positive refracting power, a fourth lens unit having negative refracting power, and a fifth lens unit having positive refracting power, the lens units being disposed in the foregoing sequential order when viewed from the object. The second lens unit is moved toward the image surface to vary the power from the wide angle end to the telescopic end. Change in the image surface occurring when the power is varied is corrected by moving the third and fourth lens units. Focusing is performed by moving at least any of the second, third and fourth lens units along the optical axis.
In general, if a zoom lens employs the rear focusing method, the size of the overall lens system can be reduced, focusing can be completed further quickly and further close-up photographing can be performed.
On the contrary, excessive aberration change occurs at the time of performing focusing, and there arises a problem in that a difficulty arises in reducing the size of the overall lens system from an infinite object to a near object while obtaining excellent optical performance.
If the zooming method is employed in which the three lens units are moved when the power is varied as disclosed above, the zooming mechanism will be undesirably complicated and the size of the lens mount cannot be reduced.
The zooming method, as disclosed in Japanese Patent Laid-Open No. 5-215967, in which the fifth lens unit, located near the image surface when the power is varied, is moved, causes the mechanism of the solid-state image sensing device, such as a CCD, and that near the film to be complicated. If dust adheres to the lens surface adjacent to the image surface, the dust image is undesirably taken on the film or the like and the image quality deteriorates.